1. Field of the Invention
The present invention relates to improvements in a directional coupler formed on a dielectric plate.
2. Description of the Prior Art
Directional couplers of such a kind as shown in FIGS. 1A, 1B and 2A, 2B have been proposed. In each of these figures indicated at 1 is a main line, at 2 is an auxiliary line, at 3 is a ground member and at 4 is a dielectric plate. In the directional coupler of FIGS. 1A and 1B the main line 1 is formed of a coupling part 1a and arms 1b extending diagonally from the opposite ends of the coupling part 1a. The auxiliary line 2 is formed likewise. In the directional coupler of FIGS. 2A and 2B the main line 1 is formed of a coupling part 1a and arms 1b extending from the opposite ends of the coupling part 1a practically perpendicularly to the coupling part 1a and a chamfered part 1c is formed in each arm 1b at the junction of the arm 1b and the coupling part 1a. The auxiliary line 2 has the same construction as that of the main line 1. Thus the directional couplers of the prior art shown in FIGS. 1A, 1B and 2A, 2B have substantially the same constructions except the morphology of the main lines 1 and the auxiliary lines 2.
Generally, directional couplers as shown in FIGS. 1A, 1B and 2A, 2B are required to meet the following conditions for satisfactory directivity and high degree of coupling:
Condition 1: Equality of the odd mode effective specific inductive capacity E.sub.odd and the Even mode effective specific inductive capacity E.sub.even of the coupling line for satisfactory directivity: and
Condition 2: Large electrostatic capacity between the coupling lines for high degree of coupling.
However, since the electric field leaks more in the odd mode than in the even mode in the conventional directional coupler, the conventional directive coupler was unable to meet the Condition 1 and had an unsatisfactory directivity. Reduction in the distance between the lines by increasing the thickness of the conductive strip contributes to increasing the electrostatic capacity between the lines to meet the Condition 2, however, the maximum degree of coupling thus obtained is, at the most --6dB, which is not satisfactory.